A commensurate-incommensurate phase transition in iron-bearing åkermanites |
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Authors: | F Seifert M Czank B Simons W Schmahl |
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Institution: | 1. Bayerisches Geoinstitut, Universit?t Bayreuth, Postfach 101251, 8580, Bayreuth, West Germany 2. Mineralogisches Institut, Universit?t Kiel, Olshausenstr. 40, 2300, Kiel, West Germany
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Abstract: | Synthetic melilites on the join Ca2MgSi2O7 (åkermanite) — Ca2FeSi2O7 (iron åkermanite) with Fe/(Fe+Mg) from 0.0 to 0.7 exhibit, at room temperature, an incommensurate phase with a rectangular modulation of a wavelength of about 19 Å in the 110] direction. Upon increase of temperature, they transform to a commensurate melilite structure at about 80° C for Fe/(Fe+Mg)=0.0 and about 250° C for Fe/(Fe+Mg)=0.6. In addition to the T(2) positions of the melilite structure filled by Si, the incommensurate phase exhibits two distinguishable T(1) sites containing the Mg and Fe2+. These two sites merge into one site during the phase transition from the incommensurate to the commensurate phase. A structural model for the incommensurate phase is based on the misfit between the tetrahedral (Mg, Fe2+)Si2O 7 4? sheets and the Ca2+ ions. |
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